A liquid delivery pump configured to deliver a liquid by reciprocating a plunger in one direction is provided with a check valve for preventing a backward flow of a liquid at an inlet or an outlet of a pump chamber. A typical check valve includes a spherical valve body arranged so as to be movable in a flow direction of a liquid and a valve seat for seating the valve body. The valve seat is a hollow cylindrical member allowing a liquid to pass through the inside thereof. When the spherical valve body is seated on the edge of the opening of the valve seat, the flow passage of the liquid is closed.
In the check valve positioned on the inlet side of the pump chamber, when the plunger is driven in the direction in which the plunger is pushed into the pump chamber (hereinafter referred to as “discharge direction”), the inside of the pump chamber becomes positive in pressure (a state higher in pressure than the atmospheric pressure). As a result, the valve body is moved toward the valve seat by the pressure of the pump chamber and seated on the valve seat. To the contrary, when the plunger is driven in the direction in which the plunger is pulled out from the pump chamber (hereinafter referred to as “suction direction”), the inside of the pump chamber becomes negative in pressure (a state lower in pressure than the atmospheric pressure). As a result, the valve body is pulled toward the pump chamber by the negative pressure and detached from the valve seat. The check valve positioned on the outlet side of the pump chamber is operated in the opposite way.
As mentioned above, in the check valve in which the valve body is passively operated by the pressure in the pump chamber, when the valve body and the valve seat are stuck with each other by the adhesion of dart, etc., to the valve body surface or the contact surface between the valve body and the valve seat, the check valve positioned on the inlet side would be no longer opened only by the negative pressure generated in the pump head at the time of the pump suction, which may sometimes result in poor liquid delivery.
For this reason, it has been proposed to actively operate the valve body by using a solenoid or the like (see Patent Document 1). By actively operating the valve body, even when the valve body and the valve seat are stuck with each other, the valve body can reliably be detached from the valve seat, so that the valve can be opened.